Medical comorbidities are common in patients with bipolar (BP) disorder but have not been previously examined for differences between United States and Europe. More than 900 outpatients with BP I and BP II disorder (mean age, 41 years) filled out a questionnaire including the occurrence of 30 listed medical conditions. The patients from the United States were from Los Angeles, Dallas, Cincinnati, and Bethesda, whereas those from Europe were from Utrecht, Freiberg, and Munich. Those from the United States had a significantly higher incidence of obesity and nine other medical comorbidities than those from Europe, who had only more cases of hyperthyroidism. The burden of medical comorbidities in patients with BP disorder from the United States seems higher than in patients from Europe. Given the adversities, morbidity, and early mortality associated with these conditions and their interaction with the morbidity and lethality of BP disorder itself, greater efforts at treatment and prevention of these medical comorbidities would seem indicated.